1. Field of the Invention
The present invention relates generally to code division multiple access (CDMA) communication systems. More particularly, the present invention relates to a CDMA communication system which utilizes virtual locating of a fixed subscriber unit to reduce the time for a base station to detect an access signal from a subscriber unit and establish a communication channel between the base station and the subscriber unit.
2. Description of Related Art
Most widely used conventional telecommunication systems require transmissions to be confined to a separate frequency or time slot. Systems using frequency division multiple access (FDMA) assign each user a specific portion of the frequency spectrum for communication. Systems using time division multiple access (TDMA) assign each user a repeating time slot to transmit the desired information. These conventional techniques require strict definition of time slots, channels and guardbands between channels in order to prevent communicating nodes from interfering with one another.
Channelization and guardband requirements have resulted in a tremendous inefficiency in the use of the RF spectrum. As the number of commercial applications of wireless technology increases, the need for communication systems which utilize the RF spectrum more efficiently has become paramount.
CDMA communication systems have had a long history of use in military applications. CDMA permits communications which are difficult to detect by enemies and offer robust communications during attempts by enemies to jam communications. In CDMA communications, each signal or communication channel is distinguished from all others in a particular frequency band by a unique pseudo noise (PN) code imprinted upon data transmitted by the transmitter. A receiver which is privy to the unique code uses the code to resolve the desired data signal channel from among many the simultaneous data signals and channels in the frequency band.
The features that have enabled CDMA communication systems to succeed in military applications also make CDMA communication systems well adapted for efficiently utilizing the RF spectrum. Since each subscriber unit in a CDMA communication system transmits and receives resolvable communication signals over the same frequency band, there are less stringent channelization and guardband requirements. Accordingly, the capacity of the system (the number of users able to communicate simultaneously) is significantly increased.
Although use of the same portion of the RF spectrum by a plurality of subscriber units increases system efficiency, each subscriber unit receives communication signals that do not have its unique code as interference. The more power that is utilized by a single subscriber unit to communicate with the base station, the more interference is presented to other subscriber units. The power from one subscriber unit may even terminate other communications if it becomes too high. Accordingly, the control of the transmission power of all subscriber units is important to maintain high quality communications throughout the system.
A typical CDMA communication system is shown in FIG. 1. The system comprises a cell base station (B), and a plurality of fixed subscriber units S1-S7 located at various distances from the base station. The base station constantly transmits a forward pilot signal. The subscriber units maintain epoch alignment between the forward pilot signal and their internal PN code generator such that all signals transmitted from the subscriber unit are at the same PN code phase at which the forward pilot is received. The base station receives signals from subscriber units with a code phase difference between its forward pilot signal and the received signal corresponding to the two-way signal propagation delay between the base station and the subscriber.
For the base station to detect a signal, it must align the phase of its receive PN code generator to the phase of the received signal, thus "acquiring" the signal. The base station can receive an access signal with any code phase difference within the range of the cell. Therefore, the base station must test all code phases associated with the range of possible propagation delays of the cell to acquire the access signal.
Once a communication channel is established between the base station and the subscriber unit, the transmission power of the subscriber unit is controlled by a closed loop automatic power control (APC) algorithm which prevents the power from each subscriber unit from excessively interfering with other subscriber units. During channel establishment, before the closed loop power control begins, the subscriber unit's transmission power is kept to a minimum by ramping-up from a low level and establishing the channel without the subscriber unit significantly overshooting (on the order of less than 3 dB) the minimum power necessary to operate the channel.
To establish a channel, each subscriber unit transmits a PN coded access signal for detection by the base station. The base station acquires the access signal and transmits a confirmation signal to each subscriber unit. The time required for the base station to acquire the access signal contributes directly to the time elapsed between a subscriber unit going "off-hook", establishing a communication channel, connecting to the public switched telephone network (PSTN) and receiving a dial tone. It is desirable to receive a dial tone within 150 msec of detection of "off-hook".
The time distribution of acquisition opportunities is shown in FIG. 2 for a typical subscriber unit located 20 km from a base station in a 30 km cell. For a base station which tests 8 code phases simultaneously at a PN rate of 12.48 MHz and a symbol rate of 64,000 symbols per second, and takes an average of 7.5 symbol periods to accept or reject a particular group of code phases, the average time to test all code phase delays within the cell is approximately 37 msec, and any one subscriber unit can only be detected during an approximately 100 .mu.sec window during that period. Assuming that the selection of initial subscriber unit transmission power level is 15-20 dB below the proper level and a slow ramp-up rate of between 0.05 and 0.1 dB/msec, it could take 4-5 such 37 msec time periods, (or an average of approximately 200 msec,) for the base station to acquire a subscriber unit. This introduces an unacceptable delay in the channel establishment process which should be less than 150 msec.
Accordingly, there is a need to reduce the amount of time required for a base station to acquire a subscriber unit.